A CO2 gas slab laser has two spaced-apart parallel rectangular electrodes separated by a relatively small distance. The electrodes are located in a housing containing a lasing gas mixture including carbon dioxide (CO2) and inert gases such as nitrogen (N2), helium (He) and xenon (Xe). Radio frequency (RF) power is applied to the electrodes and creates a gas discharge in the lasing gas mixture in the space (discharge region) between the electrodes. Ionized gas in the discharge region provides a gain medium of the laser.
The optimum distance separating the electrodes is dependent on the pressure of the lasing gas mixture, the composition of the mixture, the wavelength of the laser and the RF frequency (radio frequency). Typical electrode separation distances are between about 1.4 millimeters (mm) and 2.1 mm for RF power frequencies between about 80 megahertz) MHz and 100 MHz, and for gas mixture pressures between about 50 and 120 Torr. The output power of CO2 slab lasers scales with the area of the discharge between the electrodes. Output is usually repetitively pulsed output.
A CO2 slab lasers normally has two rectangular concave mirrors arranged to form a negative branch unstable resonator. One mirror is placed near each end of the two rectangular electrodes with one mirror (the output coupling mirror) being shorter than other (the return mirror). The negative branch unstable resonator has hybrid mode of oscillation. One mode is a guided wave mode that is guided down the length of the electrodes between opposite facing surfaces thereof. The other mode component is a free-space mode, formed by the mirrors, which zigzags between the mirrors through the discharge region until the radiation exits the resonator by bypassing the output coupling mirror. The difference in length of the mirrors determines the amount of output coupling. An output coupling of 12 to 15% is typical.
The resonator mirrors are typically made from gold plated copper (Cu). Reflecting surfaces are thin film coated for high reflectivity at the CO2 laser wavelength, which is usually 10.6 micrometers (μm), 10.3 μm, 9.6 μm, or 9.3 μm. The metal mirrors are normally mounted on aluminum end flanges of the laser housing which also serves as RF electrical ground. This mirror mounting arrangement places the metal mirrors at ground potential. One of the electrodes is the live or “hot” electrode and is insulated from the housing. The other electrode is the ground electrode and is grounded to the housing.
The mirrors should be located far enough away from the hot electrode to avoid any of the following possibilities: a discharge forming between the hot electrode and the mirrors; attracting energized ion species, generated in the discharge, to the mirror; or overexposing the mirrors to intense UV radiation which is generated within the discharge. All three possibilities can damage the mirrors, thereby causing reduced laser performance from the standpoint of output power, beam quality, and lifetime.
Typically the electrodes are separated and maintained parallel to each other by ceramic-strip spacers placed along the edges and down the length of the electrodes. When properly arranged, the ceramic spacers between the electrodes also provide beam pointing stability improvements. A problem with such spacers is that if laser power is sufficiently high, for example, about 3000 Watts (W) peak and 1000 Hz pulse repetition frequency (PRF) the ends of the ceramic spacers can be ablated by stray laser radiation, generating ceramic particles which can be deposited on the mirrors. Such deposits cause sites for further laser radiation damage.
Methods of protecting resonator mirrors from damage by oxidizing species generated in the discharge are described in U.S. Pat. No. 5,216,689, the disclosure of which is incorporated herein by reference. Here a recombination surface is formed by extending the length of the hot electrode by means of a dielectric addition thereto, with the hot electrode and dielectric extension being equal to the length of the ground electrode. Adding this dielectric extension extends the waveguide region between the metal electrodes. The discharge stops where the hot metal electrode ends. Consequently, the extended waveguide region has no gain. The '689 patent also discloses a shield having an aperture therein to pass the lasing modes.
The methods described in the '689 patent successfully address the problem of damage to the laser mirrors by the oxidizing species but do not address the problem of damage from particulates ablated from the ceramic spacers, as laser power was not high enough for such ablation to occur. The ablation problem needs to be addressed to provide reliable high-power CO2 slab lasers with commercially acceptable lifetime.